1. Field of the Invention
The present invention relates to an improved chemical vapor deposition process, such as a process for the deposition of tungsten suicide (WSi.sub.x) from tungsten hexafluoride (WF.sub.6) and dichlorosilane (DCS). More particularly, the present invention relates to a process including a novel silane (SiH.sub.4) purge step subsequent to deposition of WSi.sub.x on a substrate. The present invention further relates to a process including a novel SiH.sub.4 soak step prior to deposition of WSi.sub.x on a substrate.
2. Description of the Related Art
Tungsten silicide (WSi.sub.x) thin films have been deposited by low pressure chemical vapor deposition (LPCVD) onto semiconductor substrates using silane (SiH.sub.4) and tungsten hexafluoride (WF.sub.6) as the precursor gases. Typically, the WSi.sub.x thin film is deposited onto a semiconductor wafer having a layer of silicon oxide beneath a polysilicon layer. The foregoing process, however, has proven less than completely satisfactory.
One problem with the foregoing process is that the deposited costing is not as conformal over stepped topographies as is desired. Another problem is that films so deposited have a high residual fluorine content that adversely affects device performance. For example, when the wafer is exposed to elevated no temperatures, e.g., about 850.degree. C. or higher, as during annealing, the excess fluoride ions migrate through the underlying polysilicon layer and into the underlying silicon oxide layer. The effective thickness of the silicon oxide layer thus appears to increase. This effective thickness increase in turn leads to an adverse change in electrical properties of semiconductor devices including such layers.
When using a multichamber vacuum processing system such as that described in U.S. Pat. No. 4,951,601, to Maydan et al., incorporated herein by reference, the substrate to be coated with tungsten silicide first is cleaned using a fluorine plasma scrub to remove native oxide from the polysilicon layer. The cleaned substrate is then transferred into a substrate transfer chamber. This transfer chamber has a nitrogen or argon atmosphere (subatmospheric) to prevent re-oxidation of the substrate, and contains a robot to transfer the substrate into a processing chamber, e.g., a tungsten deposition chamber, through a slit valve having an O-ring seal. This CVD process has become the standard for depositing tungsten silicide from SiH.sub.4 and WF.sub.6. However, as substrates become larger, and feature sizes for devices become smaller, the above problems of step coverage and residual fluorine using this deposition process have become critical limitations for future applications.
An improved process for depositing WSi.sub.x films using dichlorosilane (DCS) instead of SiH.sub.4 has been proposed. The resultant WSi.sub.x films have reduced fluorine content and are more conformal than those deposited using SiH.sub.4 as the precursor gas, thereby providing a solution to the SiH.sub.4 -based deposition process limitations. In order to deposit WSi.sub.x films which have good conformality, low fluoride content and good adhesion to a substrate such as a silicon wafer (which can have one or more layers thereon), it has been found beneficial to exclude nitrogen from the deposition chamber during the deposition process. Such an improved process is provided by copending application Ser. No. 08/136,529, filed Oct. 14, 1993 by Chang et al., which is incorporated herein by reference. In this process (the "DCS process"), tungsten silicide thin films are made by passing WF.sub.6, DCS and a noble carrier gas into a tungsten deposition chamber from which nitrogen is excluded.
In deposition processes, it is customary to purge the deposition chamber and gas delivery lines after each semiconductor wafer is processed in order to remove residual reactive and carrier gases from the chamber and the delivery lines. The DCS process described above typically includes a purge step employing DCS as the purge gas.
It has been found, however, that depositing WSi.sub.x on semiconductor wafers according to the DCS process is associated with a noticeable downward drift in the sheet resistance of wafers so processed, over both short and long terms. Using the DCS process, the sheet resistance has been observed to decrease by 2 .OMEGA./square over the course of processing 25 wafers. This short-term resistivity drift amounts to a decrease of 5% or more. A long-term resistivity drift over the course of processing 500 wafers, amounting to 4-5 .OMEGA./square, has also been observed.
A need exists for an improvement in the known DCS deposition process which reduces the observed short- and long-term downward drifts in sheet resistivity.